Conventional skin treatment products are often liquid or viscous or semi-solid products, for example in the form of lotions or creams, and are traditionally applied by gentle massage or rubbing-in with the fingers. Because of the necessity for relatively large amounts of adjunct material, i.e. other than the one or more skin treatment actives which it is desired to deposit, to create an aesthetically acceptable product these known delivery systems are relatively elaborate, wasteful of cosmetic raw materials and have limited efficiency in delivering a desired active to an intended site. Control over applied dose is difficult and limited and the application of the product itself is often time consuming and messy.
As a further consequence of the presence in such products of significant amounts of stabilising ingredients such as surfactants, polymers, preservatives etc, sensory properties may often be poor, for example stickiness, greasiness and possibly irritation may be experienced by a user. This may be particularly pronounced where skin is damaged or diseased, in which circumstances application of a treatment agent by massage or rubbing-in will often be particularly undesirable.
The skin is in fact a very complex material and has many important characteristics which must be considered in the design of an optimised system for delivering cosmetic or therapeutic actives thereto. Skin has a multi-faceted surface having both lipophilic and lipophobic character, which for example allows the skin to "breathe" and release water vapour therefrom, yet function as an effective barrier against water, dirt and other unwanted materials. One particularly important physical feature of skin is its very rough surface terrain, which creates a problem in successfully applying a desired skin treatment active with 100% and even coverage.
In addition to the above described systems for delivering skin treatment agents, there are a small number of known examples where a skin treatment active is delivered using an aerosol spray. Two such examples are sprays for treating sunburn and sprains or other sports injuries. However, aerosol sprays, as are per se well known in the art for delivering personal products, also suffer from several disadvantages. For instance, the types of product and active agent which lend themselves to effective aerosol spraying are limited and the use of aerosols themselves still results in significant inefficiency and waste through non-target specific application and loss of active material to the atmosphere, which also results in unwanted atmospheric mists and possible contamination to the user's eyes, face or other body parts, which may present respiratory or other health problems. Aerosol spraying is also noisy and it is usually necessary to employ propellants which are frequently volatile organic compounds, which are now well recognised as being environmentally unfriendly, possibly hazardous to health and indeed are being legislated against in many countries of the world. The use of aerosols to deliver skin treatment agents is also believed to be even less efficient than conventional massage or rubbing-in delivery regimes in the context of percentage and evenness of coverage of the rough skin surface.
In a very different technical field, the principle of electrostatic spraying of liquid and solid materials is also known. In this technique a formulation to be sprayed is raised to a high electric potential in a spray nozzle to cause the formulation to atomise as a spray of electrically charged droplets. Such electrically charged droplets seek the closest earthed object to discharge their electric charge, and this can be arranged to be the desired spray target. Hitherto, electrostatic spraying techniques have been proposed principally for only large-scale industrial and agricultural applications, especially for delivering reactive materials like paints, adhesives and other surface coatings, as well as large-scale delivery of pesticides and other agricultural or agrochemical formulations. Examples of disclosures in this field include GB-A-1393333, GB-A-1569707, GB-A-2092025, EP-A-029301, EP-A-253539 and WO-A-85/00761, the contents of which disclosures are incorporated herein by reference.
More recently, there have been a small number of proposals for utilising the known principle of electrostatic spraying for delivering particular materials in specific applications other than those mentioned above.
EP-A-224352 suggests the use of an electrostatic sprayer for delivering a pharmaceutically active agent to the eye, to replace conventional ocular treatment using eye drops.
JP-A-56-97214 (dating from 1981) suggests the use of electrostatic spraying for applying a granular (i.e. solid particles of) colouring material to hair to effect surface coating thereof.
Also to be mentioned, though of less relevance, is U.S. Pat. No. 4776515, which proposes an electrodynamic fine particle negative ion generator adapted to spray various liquids, particularly water, but possibly also alcohol, perfume, ammonia, liquid medications and surfactants. The object of the disclosed system is to provide an ozone-free mist of negatively ionised liquid particles, (which presupposes that the material to be sprayed is ionizable), and the mist that is produced instantly disperses into an open area in which the apparatus is operated, e.g. a room, so that a far-reaching, uniform aerosol is generated which has particular applicability for large public areas such as hospitals, restaurants and offices. Clearly, this system is unsuitable for small-scale personal use and in many of its objects goes directly against the principles upon which a solution to the above mentioned prior art problems must be founded.